Question
Would you recommend planting knockout roses among hybrid roses?  

My father had a rose garden(floribunda & hybrid tea roses) which I am having
trouble maintaining and now has several open spaces.  Could I fill in with
knockout roses??

Answer
Knock Out roses grow like a floribunda so they would fit in very well with the hybrid teas. However it does depend where you garden. If you are in  a warm climate, these Knock Outs can grow very tall so you would have to keep an eye on them and prune them back hard every spring. But in a cold  winter climate, they usually grow to about 3 feet tall and wide.